A flat-panel CRT display is conventionally formed with a baseplate (or backplate), a transparent faceplate (or frontplate) that presents a desired image in the display's active area, and an outer wall that connects the baseplate and faceplate together outside the active area. The CRT display is maintained at a very low internal pressure, typically a vacuum level of 10.sup.-6 torr or less. A group of spacers, typically in the shape of walls, are often situated between the two plates inside the outer wall. In addition to maintaining a uniform spacing between the plates, the internal spacers provide the display with strength to resist external forces, such as air pressure, that could otherwise collapse the display.
Electron-emissive elements are situated in an array along the interior surface of the baseplate. A phosphor coating divided into a corresponding array of separate phosphor regions is situated along the interior surface of the faceplate. An anode is also situated over the faceplate next to the phosphor regions. During display operation, the electron-emissive elements emit electrons that are drawn by the anode towards the phosphors. Upon being struck by the oncoming electrons, the phosphors emit light that produces an image on the exterior surface of the faceplate at the front of the display. The display is controlled so that only electrons emitted from selected electron-emissive elements strike the phosphors.
More particularly, the electrons emitted from each electron-emissive element are intended to strike only one associated phosphor region. However, some of the emitted electrons invariably impinge on portions of the faceplate outside the target phosphor region. To improve the image contrast at the faceplate, a matrix of dark, largely black, non-reflective material that emits substantially no light upon being struck by electrons is suitably situated around the phosphor regions. In a color flat-panel display, this black matrix inhibits undesired mixing of colors and improves the color purity.
The black matrix can be formed in various ways. Commonly, a layer of very dark material, such as black chromium, is deposited over the interior surface of the faceplate. The dark material is typically converted into the black matrix by patterning the material using a suitable mask provided over the outer surface of the material.
It is usually desirable that the above-mentioned internal spacers not be visible on the front of the display. Accordingly, the spacers commonly overlie portions of the black matrix and thus are outside the specific portions of the active area that present the image. When the internal spacers are in the shape of walls, mechanisms such as wall grippers can be employed to hold the spacer walls in the desired locations.
U.S. Pat. No. 5,543,683 discloses a process for fabricating a black matrix and wall grippers over a faceplate of a flat-panel CRT display that utilizes internal spacer walls. In U.S. Pat. No. 5,543,683, black chromium is deposited on the faceplate and patterned using a photoresist mask to provide a black matrix function. The wall grippers are created from photo-polymerizable polyimide material formed over the black chromium. The polyimide is patterned by exposing certain polyimide portions to ultraviolet ("UV") light through a photomask placed over the outer polymide surface--i.e., the polyimide surface furthest from the black chromium--and removing the unexposed polyimide with a developer. The UV light enters the polyimide through its outer surface and causes polymerization to occur in the exposed polyimide to a specified exposure depth. The extent of polymerization is greatest at the outer polyimide surface and decreases with increasing distance from the outer polyimide surface.
Unfortunately, the polyimide thickness in U.S. Pat. No. 5,543,683 inevitably varies from point to point. At locations where the polyimide is thickest, the polyimide furthest from the outer polyimide surface--i.e., the polyimide directly along the black chromium--may be at a distance exceeding the exposure depth of the UV light and therefore may not undergo significant polymerization despite being in the line of sight of the UV light. During the development operation, the polymerized polyimide overlying the unpolymerized polyimide can be washed away, resulting in a damaged wall-gripping capability. The variation in polyimide thickness can also result in non-uniformity in display brightness.
Even if all the polyimide within the line of sight of the UV light undergoes polymerization, including all the polyimide at the locations of thickest polyimide, the polyimide furthest from the outer polyimide surface does not polymerize as greatly as the polyimide closer to the outer polyimide surface. As a result, wall grippers formed at the locations of the thickest polyimide are normally weaker than wall grippers formed at the locations of thinnest polyimide. Compared to strong wall grippers, weak wall grippers do not maintain the desired wall positions as well. Use of the technique described in U.S. Pat. No. 5,543,683 to form the black matrix and wall grippers can lead to a damaged display and/or poor display performance.
Some electrons that impinge on the phosphor regions in a conventional flat-panel CRT display are scattered rather than being collected by the anode. Part of the scattered electrons harmlessly strike the black matrix. However, others degrade display performance by striking unintended phosphor regions or charging the spacer walls. Increasing the height of the black matrix increases the percentage of scattered electrons that strike the black matrix, thereby reducing the percentage that degrade display performance. The net result is improved display performance.
In fabricating a flat-panel display, it would be desirable to have a technique for creating a black matrix from photo-polymerizable material in such a way that the black matrix is tall and adheres well to the structure on which the black matrix is formed. It would also be desirable to provide the black matrix with features for constraining the movement of spacers such as spacer walls. The spacer-constraining features should be of largely the same strength despite variations in the thickness of the photo-polymerizable material used to make the black matrix and spacer-constraining features.